So I've completely stripped the carbs ready for an ultrasonic bath and I found that the PO has removed all four of the Air Cut off Valves, looking at the forum posts, this would appear to be ok as long as you block the air bleed passages. what will be the result of my set-up, should I install valves or block the bleed?

FYI - I only ran the engine for a moment when I bought the bike as carb 2 was peeing petrol out of the overflow

Air cutoffs typically seve to avoid popping when you release the throttle at some rpm. However stock setup CB650's were never observed to pop in this situation so Honda skipped them for most PD carbs and did not do the drilling there.

So you don't really need them and if you find a secure way to block the passages it is probably the better way to avoid separating carbs again in the next few decades...

Also carb rebuild kits that don't include them are typically cheaper and moreover much easier to find.